1. Field of the Invention
The present invention relates to alkaline secondary electrochemical generators with a zinc anode, the active mass of the anode comprising at least one conducting ceramic material. According to the invention, the electrolyte of the generator consists of a highly concentrated alkaline solution and/or the active mass of the zinc anode contains an additive consisting of at least one alkali metal or alkaline earth metal titanate. The invention also relates to the zinc anode of the generators according to the invention as well as its production process.
The present invention relates to the field of electro-chemical generators, and more particularly to the field of storage batteries.
The invention relates especially to secondary generators with a zinc anode and is intended to achieve a high level of cyclability of the zinc electrode.
2. Description of the Related Art
Zinc electrodes are well known to the person skilled in the art on account of their high performance. They may furthermore be employed in various secondary electro-chemical systems such as air-zinc, nickel-zinc and silver-zinc alkaline generators, and bromine-zinc and chlorine-zinc generators with saline electrolytes.
Zinc is an attractive anodic active material, having a highly negative redox potential of −1.25 V/NHE (Normal Hydrogen Electrode) for the pair Zn/Zn(OH)2. Zinc electrodes offer a theoretical mass capacity of 820 Ah/kg. They also for example enable theoretical mass energies of 334 Wh/kg for the pair nickel-zinc (NiZn) and of 1,320 Wh/kg for the pair zinc-oxygen to be obtained. For the Ni/Zn battery, the practical mass energy is normally between about 50 and 80 Wh/kg, the voltage however being 1.65 volts instead of 1.2 volts for the other alkaline systems.
Advantages of zinc that may be emphasised include on the one hand its non-toxic nature as regards the environment (production, use and disposal), and on the other hand its low cost, which is very much less than that of the other anodic materials of alkaline batteries (cadmium and metallic hydrides) or of lithium batteries.
However, the industrial development of rechargeable systems using zinc electrodes has encountered a serious difficulty, namely the inadequate lifetime during cycling.
The reactions that take place at the anode are the following in the case of an alkaline battery:
 chargeZn+2OH− ZnO+H2O+2e− with ZnO+H2O+2OH− [Zn(OH)4]2−discharge 
It is generally the case that the recharging of a zinc electrode from its oxides and hydroxides and zincates leads to the formation of deposits whose structure is modified with respect to their original form, and are often dendritic, spongy or pulverulent. This phenomenon occurs moreover in a very large range of current densities.
Dendritic-type deposits rapidly lead to zinc being forced through the separators and to short-circuiting with the electrodes of opposite polarity.
As regards deposits of a pulverulent or spongy type, they do not allow the reconstitution of electrodes capable of functioning in a satisfactory and durable manner, since the adherence of the active material is unsatisfactory.
In addition, the chemical reduction of oxides, hydroxides and zincates to zinc at the anode during the recharging phases is also characterised by morphological changes in the electrode itself. Depending on the modes of functioning of the batteries, various types of modifications in form of the anode are found, due to a phenomenon of non-uniform redistribution of the zinc during its formation. This may be reflected in particular by a harmful densification of the anodic active mass at the surface of the electrode, most commonly in its central zone. At the same time there is generally a reduction in the porosity of the electrode, which helps to accelerate the preferential formation of zinc at its surface.
These major drawbacks, which reduce the number of cycles that can be performed to just a few dozen—an insufficient number for a secondary system to be of economic interest—have led to very many attempts aimed at improving the deposition characteristics of the zinc during recharging, so as to raise the number of charging-discharging cycles that the generator can withstand.
Widely varying methods have been investigated so as to try and minimise or retard for as long as possible these formation defects of the zinc. Among these methods, the following may in particular be highlighted:                “Mechanical” methods aimed at reducing the dendritic formation or build-up, or of avoiding pulverulent deposits: circulation of the electrolyte and/or zinc electrode in dispersed form; subjecting the electrodes to vibrations; use of separators resistant to perforation by the dendrites, often in multiple layers, and even of ion-exchange membranes in order to prevent the migration of zincates;        “Electrical methods” intended to improve the conditions of formation of the zinc deposit: monitoring of the charging parameters (intensity, voltage, etc.); use of pulsed currents, including current inversions in order to try and dissolve the dendrites during formation;        “Chemical” and “electrochemical” methods: use of additives incorporated in the electrolyte (fluoride, carbonate, etc.) and/or in the anodic active material (calcium, barium, etc.) and dilution of the electrolyte so as in particular to limit the solubility of the zincates and to form zinc oxide and insoluble zinc compounds.        
These various techniques may be employed individually or in combination.
Their positive effects are in any case limited and are often found to be insufficient to confer any economic viability on secondary generators with zinc anodes, and in particular on the pair NiZn, which however is theoretically very attractive; they scarcely enable for example one hundred cycles carried out at discharging levels that are significant to be exceeded or even attained.
These techniques furthermore in some cases have serious negative effects, such as:                increase in internal resistance of the battery (due to certain additive or to the dilution of the electrolyte),        reduction in the lifetime of nickel cathodes (due to the use of certain additives),        mechanical complexity of functioning (for systems involving circulation),        increases in volume and mass of the systems (deterioration of the specific technical performance in terms of mass and volume energies),        increase in cost (loss of potential economic advantage).        
A major innovation was provided and described by the invention disclosed in French patent application FR 2.788.887, the elaborated technology enabling several hundreds of cycles to be performed in a large range of operating regimes and up to very high discharging levels, by virtue of the use of means intended to increase the utilisation factor of the active material by improving the percolation of the charges within it.